The present invention concerns a method and system for the OFDM orthogonal frequency division multiplexing multicarrier transmission of digital broadcasting signals, in particular for digital audio broadcasting and for digital value-added services.
The DAB (digital audio broadcasting) transmission system was developed over the past few years for terrestrial digital audio broadcasting. The known system is suitable for transmitting high-quality audio programs to mobile, portable, and stationary receivers. It is also generally possible to transmit additional data at relatively low data rates over the DAB transmission system, for example information accompanying the program or traffic information. Multiple audio programs and data services are combined into a DAB ensemble and broadcast together at a transmit frequency using a selected coded orthogonal frequency division multiplexing (COFDM) method. The known DAB transmission system can currently be used to transmit a maximum effective net data rate of 1728 Kbps. Due to rapid developments in the multimedia field, there is great interest in transmitting added-value services, such a video programs, at higher data rates, which can exceed the current effective net data rate of 1728 Kbps. However, the conventional DAB system is unsuitable, in particular, for mobile reception of higher-speed data, since the error protection mechanism used is mot sufficiantyl effective.
An object of the present invention is therefore to further develop the existing DAB transmission system so that added-value services, such as video programs, can be transmitted at higher data rates without impairing the transmission quality.
The present invention provides a digital OFDM multicarrier transmission system that is based on the existing DAB transmission system with improved system performance. At the same time, attention was paid to the ability to use known hardware components from the existing DAB transmission system when designing the digital OFDM multicarrier transmission system, making it possible to easily incorporate the present invention into an existing DAB transmission system. For convenience, the transmission system according to the present invention is denoted as the X-DAB transmission system. xe2x80x9cX-DABxe2x80x9d stands for extended, or enhanced, digital audio broadcasting system. It should be noted at the outset that the X-DAB transmission system can be used to advantageously transmit high-speed data signals, such as video signals, to mobile receivers along with audio programs, even though this cannot be done with a conventional DAB transmission system at a sufficient level of quality.
With the method according to the present invention for the OFDM multicarrier transmission of digital broadcasting signals, at least one source data stream, which is split into several frames of a predetermined length, is generated, as with the known DAB system. In order to considerably improve system performance compared to the DAB transmission system, the source data stream is broken down into N parallel data substreams, each of which is supplied to a separate channel encoder having a predetermined code rate. Each channel encoder supplies an encoded, preferably convolution-encoded, sequence of M bits at its output. The bits of the N encoded parallel data substreams are each combined into an N-tuple, i.e., a group or a vector of N bits, and mapped to a complex symbol of a 2N-PSK symbol alphabet. This encoded modulation technique is essentially known. A difference compared to the existing DAB transmission system is that the X-DAB transmission system processes complex symbols or their N-bit addresses instead of individual bits immediately after channel encoding of the source data stream. It has been determined that the improved system characteristics are, in fact, due to this measure. Consequently, complex symbols, and not individual bits, are combined into blocks of a predetermined size. As with the known DAB transmission system, in which, however, bits are mapped to complex symbols only after blocks are generated, the complex symbols in each block are each assigned to different subcarriers. An analog OFDM signal is then generated from the complex symbols in each block and transmitted to receiving equipment.
To avoid transmission errors caused by the time-discriminating characteristics of a mobile radio channel, the complex symbols are time-interleaved prior to block generation. Note that individual bits, and not complex symbols, are time-interleaved in the known DAB transmission system. To eliminate signal impairment caused by a frequency-discriminating mobile radio channel, the complex symbols in each block are frequency-interleaved after block generation, which is also the case in a known DAB broadcasting system. The goal of time- and frequency-interleaving is to transmit adjacent signal elements as far apart from each other as possible, thereby avoiding grouping errors in adjacent information elements.
The complex symbols in each block undergo an essentially known differential modulation on each subcarrier.
In contrast to the known DAB transmission system, in which the subcarriers in each block undergo a 4-PSK modulation, the X-DAB transmission system according to the present invention carries out a 2N-PSK modulation on the subcarriers, with N being set to a value greater than or equal to 3. Although the present invention uses a higher modulation method than the known DAB transmission system (at least 8-PSK), the system quality does not deteriorate, as would be expected, with a constant signal-to-noise ratio on the receiver side. This is due to the symbol mapping function, which is performed earlier than in the known DAB transmission system.
In addition to improved system characteristics, the digital OFDM multicarrier transmission system according to the present invention is characterized by its downward compatibility with the conventional DAB system. Downward compatibility means that the X-DAB transmission system according to the present invention can be embedded into an existing DAB transmission system, making it possible to transmit DAB programs and X-DAB programs in a shared transmission frame. The ability of the two transmission systems to coexist is achieved by using the same parameters for the OFDM method, including those for frequency interleaving and differential modulation. In order to minimize the additional hardware and software needed for implementing the OFDM multicarrier transmission system according to the present invention, the convolution encoder, convolution decoder, time interleaver, and time deinterleaver known from the DAB system are also used.
A receiving device, which is initially designed like a DAB receiver, is provided for receiving an OFDM signal. The known modules include an OFDM demodulator with an A/D converter and a device for performing a discrete Fourier transform of the OFDM signal, a differential demodulator, and a frequency deinterleaver. Instead of converting the OFDM signal to a bit stream and subsequently supplying it to a device for removing the block structure, as in the case of known DAB receivers, the complex symbols themselves are supplied to a device which eliminates the block structure. The stream of complex symbols is supplied to a demultiplexer for time-division demultiplexing of the complex symbols from different source data streams. A time deinterleaver reverses the time interleaving of the complex symbols. In contrast to a known DAB receiver, the data streams undergo channel decoding on the complex symbol level, and not bit-by-bit. For this purpose, the complex receiving symbols are applied to N parallel-connected metric calculators. A convolution decoder is connected downstream from each metric calculator. The convolution decoder outputs are fed back to the metric calculators via assigned complementary convolution encoders according to a selected symbol mapping rule executed on the transmitter side, such as a natural mapping or pragmatic mapping rule. The decoded data substreams are present at the output of each convolution decoder. The individual data substreams are supplied to a source decoder for further processing.